(1) Field of the Invention
The present invention generally relates to a cylinder block of an internal combustion engine, and more particularly to a cylinder block in which crank caps are bolted and a crankshaft is rotatably supported by the crank caps on crank journals.
(2) Description of the Related Art
As disclosed in Japanese Laid-Open Patent Application No.5-157005, there is known a cylinder block in which crank caps are bolted and a crankshaft is rotatably supported by the crank caps on crank journals. The cylinder block includes a plurality of cylinder bores corresponding to respective cylinders of an internal combustion engine. In the cylinder bores, pistons of the engine are capable of moving up and down during rotation of the engine, the pistons being connected to the crankshaft through connecting rods.
In the cylinder block, the crank caps are bolted to bottom portions of the cylinder block so that the crankshaft is rotatably supported by the crank caps on the crank journals of the cylinder block. The crank caps have bearing bores on which the crankshaft is supported in conjunction with the crank journals of the cylinder block.
The cylinder block includes a plurality of bulkheads which partition an internal space of a crankcase beneath the cylinder bores into sub-sections for the respective cylinders of the engine. The bulkheads includes the crank journals. In order to bolt the crank caps to the cylinder block, threaded holes are formed in a bottom of each of the bulkheads. Bolts are fastened to the threaded holes to secure the crank caps to the cylinder block.
In the cylinder block of the above-mentioned publication, the threaded holes are simply formed as closed-end threaded holes in the bulkheads. That is, the threaded holes have the ends enclosed with the internal walls of the bulkheads and not open to the internal space of the cylinder block.
Recently, in order to provide an engine having a lighter weight, cylinder blocks have come to be made of an aluminum alloy, which are different from a conventional cylinder block made of cast iron. In addition, in order to provide a good ventilation for the crankcase of the cylinder block and reduce a friction loss of the engine, breathing holes are formed in the bulkheads of the cylinder block. Further, in order to provide a compact engine having a lighter weight, a distance between two of the cylinder bores of the cylinder block in an axial direction of the crankshaft provided in the crankcase is set to be relatively small. Each of the bulkheads in the cylinder block is provided with raised portions and thin-wall portions. In each bulkhead, the raised portions are formed as having a thickness greater than a thickness of the thin-wall portions.
Generally, the bulkheads of the cylinder block on which the crankshaft is supported are subjected to explosive forces and thermal stresses produced during the rotation of the engine. Further, the bulkheads of the cylinder block are subjected to steep changes in pressure produced by the upward and downward movements of the pistons in the cylinder bores during the rotation of the engine. The corner locations between the raised portions and the thin-wall portions in each of the bulkheads are likely to suffer the concentration of the forces and stresses produced during the rotation of the engine.
As described above, the breathing holes are provided in each of the bulkheads. The breathing holes function to avoid the concentration of the forces and thermal stresses at the corners of the bulkheads produced during the rotation of the engine, and function to reduce the pressure changes of the corners of the bulkheads produced during the rotation of the engine. Consequently, the breathing holes are effective in preventing the occurrence of cracks or damages of the corner locations of the bulkheads during the rotation of the engine.
In the cylinder block of the above-mentioned publication, it is required that the threaded holes to which the bolts are fastened to fix the crank caps to the cylinder block be formed in the bulkheads. The threaded holes are formed in each bulkhead in two stages: a drilling of holes before threading; and a thread cutting of the holes in the bulkhead by using a tap.
Since the explosive forces and thermal stresses produced during the rotation of the engine are directly applied to the crankshaft provided in the crankcase of the cylinder block, it is necessary to firmly support the crankshaft on the crank journals of the cylinder block. Because of this, a length of the connection between the bolts and the threaded holes in the bulkheads has to be relatively long in order to firmly support the crankshaft on the crank journals. Therefore, it is required that the threaded holes which are relatively deep be formed in the bulkheads.
However, in the cylinder block of the above-mentioned publication, the threaded holes are simply formed as the closed-end holes in the bulkheads. It is difficult to clear away chips produced during the cutting of the threaded holes in the bulkheads. It is difficult to supply a cutting fluid to the threaded holes during the cutting thereof. Therefore, in a case of the cylinder block of the above-mentioned publication, it is very difficult to ensure accurate machining of the threaded holes in the bulkheads.
In a case in which the cylinder block is made from an aluminum alloy in order to provide an engine having a lighter weight, it is more difficult to clear away the chips produced during the cutting of the threaded holes in the bulkheads due to a high toughness of the aluminum alloy.
In order to ensure an accurate machining of the threaded holes in the bulkheads even if the aluminum alloy is used, it is necessary to decrease the cutting speed. If the cutting speed is decreased, the efficiency of the machining for the cylinder block has to be lowered. In particular, when the threaded holes which are relatively deep are formed in the bulkheads, the efficiency of the machining for the cylinder block of the above-mentioned publication will be considerably lowered if the cutting speed is decreased.